Called when a scalar value is found. The scalar may have an
anchor, a tag, be implicitly plain
or implicitly quoted

value is the string value of the scalar anchor is
an associated anchor or nil tag is an associated tag or nil
plain is a boolean value quoted is a boolean
value style is an integer idicating the string style

Called when the document starts with the declared version,
tag_directives, if the document is implicit.

version will be an array of integers indicating the YAML
version being dealt with, tag_directives is a list of tuples
indicating the prefix and suffix of each tag, and implicit is
a boolean indicating whether the document is started implicitly.

anchor is the anchor associated with the map or
nil. tag is the tag associated with the map or
nil. implicit is a boolean indicating whether or
not the map was implicitly started. style is an integer
indicating the mapping style.

anchor is the anchor associated with the sequence or nil.
tag is the tag associated with the sequence or nil.
implicit a boolean indicating whether or not the sequence was
implicitly started. style is an integer indicating the list
style.